Increasing low temperature flowability of middle distillate fuel

ABSTRACT

The low temperature flowability of a middle distillate petroleum fuel oil, boiling in the range of about 250* to about 700* F. is improved and its pour point is lowered by adding to the fuel oil from about 0.01 to about 3 wt. % of an essentially saturated hydrocarbon fraction which is substantially free of normalparaffinic hydrocarbons, and which has a number average molecular weight in the range of about 600 to about 3,000.

United States Patent Feldman et al.

[ 51 May 2, 1972 INCREASING LOW TEMPERATURE FLOWABILITY OF MIDDLE DISTILLATE FUEL Inventors: Nicholas Feldman, Woodbridge; Wladimir Philippoff, Cranford, both of NJ.

Assignee: Esso Research and Engineering Company Filed: Mar. 17, 1969 Appl. No.: 807,953

U.S. Cl.... ..44/80, 208/15 Int. Cl. ..Cl0l 1/04 Field of Search ..208/15, 33; 44/80 References Cited UNITED STATES PATENTS 9/ 1959 Farmer et al. ..208/45 3,132,083 5/1964 Kirk ..208/33 3,507,776 4/1970 l-lann ..208/15 Primary Examiner-Daniel E. Wyman Assistant Examiner-Mrs. Y. H. Smith Attorney- Pearlman & Stahl and Byron O. Dimmick 57] ABSTRACT 6 Claims, No Drawings INCREASING LOW TEMPERATURE FLOWABILITY OF MIDDLE DISTILLATE FUEL FIELD OF THE INVENTION Heating oils and other middle distillate petroleum fuels, e.g., Diesel fuels, contain normal paraffin hydrocarbon waxes which at low temperatures tend to precipitate in large crystals in such a way as to set up a gel structure which causes the fuel to lose its fluidity. The lowest temperature at which the fuel will still flow is generally known as the pour point. When the fuel temperature reaches or goes below the pour point andthe fuel is no longer freely flowable, difficulty arises intransporting the fuel through flow lines and pumps, as for example when attempting to transfer the fuel from one storage vessel to another by gravity or under pump pressure or when attempting to feed the fuel to a burner. Additionally, the wax crystals that have come out of solution tend to plug fuel lines, screens and filters. This problem has been well recognized inthe past and various additives have been suggested for depressing the pour point of the fuel oil. One function of such pour point depressants has been to change the sizeof the crystals that precipitate from the fuel oil, thereby reducing the tendency of the wax crystals to set into a gel. Small size crystals are desirable so that the precipitated wax will not clog the fine mesh screens that are provided in fuel transportation, storage, and dispensing equipment. It is thus desirable to obtain not only fuel oils with low pour points but also oils that will form small wax crystals so that the clogging of filters will not impair the flow of the fuel at low operating temperatures.

DESCRIPTION OF THE INVENTION In accordance with the present invention, it has been found that a paraffin hydrocarbon fraction, preferably a naturally occurring fraction, that is substantially free of normal paraffm hydrocarbons, i.e., containing no more than about 5 wt. preferably 3 wt. or less, and most preferably no more than about one weight percent, of normal paraffin hydrocarbons, and that has a number average molecular weight of from about 600 to about 3,000, when'added to a middle distillate petroleum fuel oil in a concentration of about 0.01 to about 3 wt. preferably about 0.1 to 2 wt. will depress the pour point of the fuel oil and will also modify the wax crystal structure and will enable the fuel to flow through screens at temperatures much lower than would be possible in the absence of the added paraffinic fraction. The active components of the normal-paraffin-free fraction can be either isoparafiins or cycloparaffins or mixtures of both types.

It has previously been taught to add a microcrystalline wax or a petrolatum to a distillate hydrocarbon fuel oil. See U.S. Pat. Nos. 3,250,599 and 3,288,577. However, the prior art teaching is that petrolatum or microcrystalline wax, when used alone, does not satisfactorily improve the low-temperature pumpability or filterability of a wax-containing petroleum distillate. Moreover, such petrolatum or microcrystalline wax does not lower the pour point, and in some cases raised the pour point of the fuel oil to which it is added. In contrast to this, it is the finding of the present invention .that an essentially saturated hydrocarbon fraction of number average molecular weight in the range of about 600 to about 3,000 that is substantially free of normal paraffin hydrocarbons can .of itself improve the low-temperature flowability of a distillate petroleum fuel oil and also lower its pour point.

While it is not intended that this invention be limited by any theory as to its operation, it is believed that, by incorporating normal-paraffin-free, essentially saturated hydrocarbons of molecular weight in the range of from about 600 to 3,000, the molecules of the normal paraffin hydrocarbons that' are present in the fuel oil are affected in such a manner that their crystallizing tendency is reduced, especially in the lateral direction. This causes the normal paraffins in the fuel to form much smaller crystals than they would otherwise. The result is that while the crystallization of the paraffin wax hydrocarbons that are present in the fuel oil is not prevented as the temperature is lowered, the shape and size of the crystals are. such that they do not interfere with the low temperature flowability of the fuel oil to the extent that they would have if they had crystallized in the normal fashion.

The distillate fuel oils that can be improved by this invention include those having boiling ranges within the limits of about 250 to about 700 F. The distillate fuel oil can comprise straightrunor virgin gas oil or cracked gas oil or a blend in any proportion of straight run and thermally and/or catalytically cracked distillates.

The most common petroleum middle distillate fuels are kerosine, diesel fuels, jet fuels and heating oils. Since jet fuels are normally refined to very low pour points there will be generally no need to apply the presentinvention to such fuels. The low temperature flow problem is most usually encountered with diesel fuels and with heating oils. A representative heating oil specification calls for a 10 percent distillation point no higher than about 440 F., a 50 percent point no higher than about 520 F., and a percentpoint of'at'least 540 F. and no higher than about 640 to 650 F., although some specifications set the 90 percent point as high as 675 F. Heating oils are preferably made of a blend of virgin distillate, e.g.,

gas oil, naphtha, etc., and cracked distillates, e.g., catalytic cycle stock. A representative specification for a diesel fuel includes a minimum flash point of F. and a 90 percent distillation point between 540 and 640 F. (See ASTM Designations D-396 and D-975) v The fractions of essentially saturated hydrocarbons that are used in accordance with the present invention as flow improvers and pour point depressants are generally amorphous solid materials having melting points within the range of about 80 to 140F. and having number average molecular weights within the range of about 600 to about 3,000. This molecular weight range is above the highest molecular weight of any hydrocarbons that are naturally present in the fuel oil.

An amorphous hydrocarbon fractionthat is useful as a fuel oil flow improver in accordance with this invention can be obtained by deasphalting a residual petroleum fraction and then adding a solvent such as propane, lowering thetemperature of the solvent-diluted residuum, and recovering the desired solid or semi-solid amorphous product by precipitation, followed by filtration. The-residual oil fractionsfrom which the desired amorphous hydrocarbons are obtained will have viscosities of at least 125 SUS at 210 F. Most of these residual oils are commonly referred to as bright stocks.

In some instances products obtained by this procedure will be naturally low in normal paraffin hydrocarbons and can be used in the present invention without further treatment. For

example, by deasphalting a residual oil from certain'Texas coastal crudes and then propane treating the residual fraction, a high molecular weight fraction can be obtained which has only a trace of normal paraftins, about 5 percent of isoparaffins, about 73 percent of cycloparafiins and about 22 percent of aromatic hydrocarbons. In other instances it is necessary to treat the high molecular weight fraction in some manner to reduce its content of normal paraffins. Removal of normal paraffins from an amorphous hydrocarbon mixture can be eff fected by complexing with urea, as will be illustrated hereinafter in one of the examples. Solvent extraction procedures can also be used, but in many instances they are not as effective as complexing techniques. Thus the amorphous hydrocarbon mixture can be dissolved in heptane at its boiling point and then when the solution is cooled to room temperature the normal paraftins will be predominantly precipitated and the resultant supernatant solution will give a mixture containing some normal paraffins but predominating in cycloparaffins a'nd isoparaffins.

Vacuum distillation can also be used for the removal of normal paraffin hydrocarbons from a high molecular weight paraffinic fraction, but such a procedure requires a very high vacuum, i.e., less than 5 mm'Hg, absolute. pressure, preferably a pressure below 3 mm Hg, absolute, e.g., 2 mm or micronsJf thepressure used is 5 mm or higher, the necessary temperature for the distillation is high enough to cause cracking of the constituents, which is undesirable.

In some instances the action of the high molecular weight hydrocarbon fraction in improving fuel oil low temperature flow properties can be enhanced by employing in conjunction with the added hydrocarbon fraction a small amount e.g., 0.001 to 1 wt. of a wax-modifying pour-point depressant of the type comprising a copolymer of ethylene with another ethylenically unsaturated monomer, wherein the ethylene forms a backbone along which there are randomly distributed side chains consisting of hydrocarbon groups or oxy-substituted hydrocarbon groups of up to 16 carbon atoms. For example, there may be incorporated into a distillate fuel oil having a distillation range of about 320 to 654 F., 0.2 wt. of the hydrocarbon fraction described in Example 1, infra, and 0.03 wt. of a copolymer of ethylene and vinyl acetate having a mole ratio of ethylene to vinyl acetate of about 4.2 and having an average molecular weight of about 1,740 as determined by vapor phase osmometry, the copolymer having been prepared at 82 C. and 750 psig pressure in the presence of dilauroyl peroxide catalyst.

The pour point depressants of the type comprising a copolymer of ethylene and at least one second unsaturated monomer can include as the second unsaturated monomer another monoolefin, e.g., a C to C alpha-monoolefin or an unsaturated ester, as for example vinyl acetate, vinyl butyrate, vinyl propionate, lauryl methacrylate, ethyl acrylate or the like. (See Canadian Patents 676,875 and 695,699). Other second monomers include N-vinyl pyrrolidone. The second monomer can also be a mixture of an unsaturated monoester or diester and a branched or straight chain alpha monoolefin. Mixtures of copolymers can also be used, as for example mixtures of a copolymer of ethylene and vinyl acetate with an alkylated polystyrene or acylated polystyrene (see U.S. Pat. No. 3,037,850 and 3,069,245).

Stated more generally, the copolymer pour depressants will consist essentially of about 3 to 40, and preferably 3 to 20, molar proportions of ethylene per molar proportion of the ethylenically unsaturated monomer, which latter monomer can be a single monomer or a mixture of such monomers in any proportion, said polymer being oil-soluble and having a number average molecular weight in the range of about 1,000 to 50,000, preferably about 1,500 to about 5,000 molecular weight. Molecular weights can be measured by cryoscopic methods or by vapor-phase osmometry, for example by using a Mechrolab Vapor Phase Osmometer Model 310A. Particularly desirable are ethylene copolymers wherein the number of methyl terminating side branches per 100 methylene groups of the copolymer does not exceed about 6. Such copolymers are prepared by copolymerizing the monomers in an inert solvent such as benzene or hexane at a temperature of about 70 to 130 C. using a free radical catalyst with a halflife'ofless than 1 hour at 130 C., e.g., di-lauroyl peroxide.

The unsaturated monomers, copolymerizable with ethylene include unsaturated acids, acid anhydrides, and mono and diesters of the general formula:

wherein R is .hydrogen or methyl; R is a -OOCR or COOR, group wherein R is hydrogen or a C to C preferably a C to C straight or branched chain alkyl group and R is hydrogen or COOR The monomer, when R to R are hydrogen and R is -OOCR includes vinyl alcohol esters of C to C monocarboxylic acids. Examples of such esters include vinyl acetate, vinyl isobutyrate, vinyl laurate, vinyl myristate, vinyl palmitate, etc. When R is -COOR., such esters include iso-octyl acrylate, methyl acrylate, methyl methacrylate,

lauryl acrylate, isobutyl methacrylate, palmityl alcohol ester of alpha-methyl-acrylic acid, mixed C isomeric alcohol esters of methacrylic acid, etc. Examples of monomers wherein R is hydrogen and R and R are OOCR., groups, include mono iso-octyl alcohol fumarate, di-isopropyl maleate; di-lauryl fumarate; ethyl methyl fumarate, fumaric acid, maleic acid, etc;

Other unsaturated monomers copolymerizable with ethylene to prepare pour point depressants include C to C branched chain or straight-chain alpha monoolefins, as for example propylene, n-octane-l, 2-ethyl decene-l, n-decene-l, etc.

Small proportions, e.g., about 0 to 20 mole percent, of a third monomer, or even of a fourth monomer, can also be included in the copolymers, as for example a C to C branched or straight chain alpha monoolefin, e.g., propylene, n-octenel, n-decene-l, etc. Thus, for example, a copolymer of 3 to 40 moles of ethylene with one mole of a mixture of 30 to 99 mole percent of unsaturated ester and 70 to 1 mole percent of olefin could be used.

Any of the known methods for polymer preparation can be used in preparing the copolymer of pour depressant, including the techniques taught for ethylene-vinyl ester polymerizations in US. Pat. Nos. 3,048,479, 3,131,168, 3,093,623 and 3,254,063. The copolymers that are formed are random copolymers consisting primarily of I an ethylene polymer backbone along which are distributed side chains of hydrocarbon or oxy-substituted hydrocarbon.

Other pour depressants that can be used include the acylated polystyrenes and the alkylated polystyrenes. (See US. Pat. No. 3,069,245)

The nature of this invention and the manner in which it can be practiced will be more fully understood when reference is made to the following examples, which include a preferred embodiment.

EXAMPLE 1 An amorphous hydrocarbon fraction (mp. 11 1 F.) obtained by propane precipitation from the deasphalted residuum of a Texas coastal crude oil was found by mass spectrographic analysis, and by gas chromatography, to contain 5 wt. of isoparaffins, 22 wt. of aromatic hydrocarbons, 73 percent of cycloparaffins, and no more than a trace of normal paraffin hydrocarbons. The number average molecular weight of this material was about 775 as determined by osmometry.

The distillation characteristics ofthis solid hydrocarbon fraction were as follows:

Distillation Vapor Temp. (ASTM Vapor Temp. Converted to D-l at 5 mm Hg Atmospheric Pressure Initial BP 442 F. 754 F.

Oil E: Boiling range 368654 F.; 80 percent Cat Cycle Stock 20 percent Virgin Naphtha.

Each of the heating oil blends and each of the heating oils with no additional additive present were subjected to two different low temperature fiow tests. One of these tests, called a PFT test (programmed fluidity test) is conducted as follows:

A 40 milliliter sample of the oil to be tested is placed in an hourglass-shaped device, having upper and lower sections connected by an opening between the two sections having a diameter of about 2.25 mm, the opening being initially closed by a thin aluminum disc. The tester containing the oil is placed in a cold box and the oil is cooled from a point 10 F. above the cloud point to a temperature ofl F., at the rate of 4 F./hr. The testeris inverted and the oil is allowed to settle for one minute. Then the aluminum disc is punctured so that the oil fiows from the upper chamber through the aforesaid opening into the lower chamber. The oil has passed the test when at least 85 volume percent of the oil flows from the upper chamber to the lower chamber in a time of 3 minutes or less.

The other low temperature flow test, called a low-temperature filterability test, is conducted as follows: A 200 milliliter sample of the oil is cooled at a controlled rate of 4 F. per hour until a temperature of either 10" F. or of 20 F. is reached, these being the temperatures at which the flow test is conducted. The oil is then permitted to flow by gravity at the test temperature through a 20 mesh screen of9 millimeters diameter for 25 seconds. The volume percentage of oil that has flowed through the screen at the end of this time is then measured. If more than 85 volume percent of the oil has gone through the screen at the end of the 25 seconds, the oil is considered to have passed the test.

The compositions of the various oil blends tested, their measured ASTM pour points, and the test results obtained in the low temperature flow tests are given in Table I, which follows. The weight percentages of the added materials are based on the total composition in each instance. It will be seen from the data that the added normaI-paraffin-free hydrocarbon fraction was quite effective in improving the low temperature properties of each of the heating oils.

TABLE I Weight percent 01 Percent recovery in flow test normztlpnmmn Filterability test. I hydroearhom PFI 20 mesh screen 011 free fraction ASTM test, used added pour pt. 10" F. 10

.0 0 1 0 E .5 E 5 1.0 25 F E 2.0 Below -35 9e EXAMPLE 2 A mixture of solid hydrocarbons obtained in the dewaxing' of a petroleum oil bright stock was found to contain 20 wt.

of normal paraffins, 24 wt. of isoparaffins, 36 wt. of 5 of toluene. The resulting solution was diluted with 300 milliliters of acetone while stirring. Powdered urea wasthen added to this diluted solution, a few grams at a time, until a total of 100 grams of urea had been added. Stirring was continued for 2 hours after the last addition of urea. The reaction produce was then filtered through a Buchner funnel using Number 1 Whatman filter paper. The slurry collected on the filter paper was washed with two 150 milliliter portions of cyclohexane and then with two 100 milliliter portions of toluene and the washings were combined with the original filtrate. The combined filtrate and washings were transferred to a separatory funnel and extracted seven times with 100 milliliter portions of water in order to remove any dissolved urea that might be present. The hydrocarbon layer in the separatory funnel was then transferred to a distillation flask and all of the solvents were removed by distillation. The distillation residue, which amounted to 16 grams, i.e., percentof the original solid hydrocarbon mixture, had a pasty appearance. The product is hereinafter referred to as non-normal-parafiin hydrocarbon mixture A, or more simply hydrocarbon mixture A. The product had a number average molecular weight of I 817 as determined by osmometry and a melting point of 1 13 F. Both the original solid hydrocarbon mixture and hydrocarbon mixture A were blended in various weight percent concentrations in fuel oil E described above. The weight percentages in each instance are based on the total composition. Each of the blends was subjected to the programmed fluidity test (PFT) and to the low temperature filterability test also described above, with the modification that the latter test was run at -5 F. using a 30 mesh screen and at 10 F. with a 20 mesh screen. The results obtained are given in Table 11 which follows:

TABLE 11 Percent recovery in flow tests Wt. percent added hydrocarbon mixture in fuel oil E Filterability test PFT Original test, 20 mesh 30 mesh mixture Mixture A 10 F. -10 F. 5 F.

I 47 n n 51 1 1 69 5 1 93 U4 4 92 87 93 100 100 The data in Table II show that the presence of appreciable amounts of normal paraffin hydrocarbons in the added hydrocarbon mixture actively interferes with the flow improvement properties of the added hydrocarbon mixture.

EXAMPLE 3 A portion of the original mixture of solid hydrocarbons of I example 2, containing 20 wt. of normal paraffins, was subjected to vacuum distillation at 2 mm Hg absolute pressure to a final still temperature of 650 F., the final vapor temperature being 583 R, which is equivalent to 965 F. at atmospheric pressure. The distillation took about 25 percent of the material, leaving about 75 percent bottoms. The bottoms product, when checked by gas chromatography, showed that no normal paraffins were present. When 0.4 wt. of the bottoms product was blended into fuel oil E, the blend gave 97 percent recovery at -10 F. through a 30 mesh screen and 100 percent recovery at 0 F. through a 40 mesh screen using the filterability test described in example 2.

EXAMPLE 4 A quantity of the solid hydrocarbon fraction described in example 1 was subjected to vacuum distillation in a molecular still at pressures in the range of from 5 to 20 microns absolute, and at temperatures ranging from 191 to 342 C. Various fractionsobtained in this vacuum distillation were tested for their effect in improving the flow properties of a fuel oil by blending 0.75 wt. of each of several of the fractions in separate portions of fuel oil E described above. These blends were then subjected to the PFT test at 10 F. Also tested was a blend prepared by adding, to fuel oil E, 0.75 wt. ofa solid hydrocarbon mixture of 75 F. melting point and 508 average molecular weight that had been obtained in the dewaxing ofa TABLE 111 EFFECT OF MOLECULAR WEIGHT OF souD HYDROCARBONS ADDED TO FUEL OIL Vacuum Average 1 Distillation Molecular Weight 1 Recovery Fraction of Fraction in PFI Test I 459 3 535 36 6 590 80 13 723 86 21 960 93 24 1083 95 Solid Hydrocarbons from 550 Neutral 508 15 It will be seen from the data in Table III that those solid hydrocarbon mixtures that had molecular weights below about 590 were not satisfactory in improving the flow properties of the fuel oil, whereas those fractions of 590 molecular weight and higher were effective.

The effect of the aromatic hydrocarbons present in the solid hydrocarbon fractions used in demonstrating this invention was found to be negligible. This is shown by the following test. The solid hydrocarbon fraction used in example I was separated into an aromatic hydrocarbon fraction (22 percent) and a non-aromatic hydrocarbon fraction (78 percent) by a silica gel separation technique wherein the the original hydrocarbon mixture was dissolved in normal heptane and the solution was percolated through a column of silica gel, the

8 column was then flushed with normal heptane, the effluent streams were combined, and the non-aromatics were recovered by evaporating off the normalheptane. The aromatics that had been adsorbed in the column were then improvement, whereas the non-aromatic-hydrocarbon fraction was about 20 percent more effective in low temperature flow improvement thanthe original hydrocarbon mixture, i.e., for a given degree of flow improvement, about20 percent less of the non-aromatic fraction was needed than with the original mixture, indicating that the aromatic hydrocarbons in the original mixture were merely exerting a dilution effect, and that the non-normal paraffms andthe cycloparaffins are the active flow improver components of the solid hydrocarbon mixture. J t I I .Wl-lAT IS CLAIMED IS ILA wax-containing petroleum distillate fuel having a boiling range within the limits of about 250 and 700? F. which has been improved with respect to its low temperature flow properties by adding thereto from about 0.01 to about 3 wt. of

a flow-improving, amorphous, normally solid essentially satu- 4. Improved fuel as defined by claim 1 wherein said fuel is a I heating oil.

5. Improved fuel as defined by claim 1 wherein said fuel is a blend of virgin distillate components and'cracked distillate components. I

6. Improved fuel as defined by claim 1 wherein said added fraction contains no more then about 1 wt. of normal paraffin hydrocarbons. 

2. Improved fuel as defined by claim 1 wherein the amount of said added hydrocarbon fraction is from about 0.1 to about 2 wt. %.
 3. Improved fuel as defined by claim 1 wherein said added fraction contains no more than about 3 wt. % of normal paraffin hydrocarbons.
 4. Improved fuel as defined by claim 1 wherein said fuel is a heating oil.
 5. Improved fuel as defined by claim 1 wherein said fuel is a blend of virgin distillate components and cracked distillate components.
 6. Improved fuel as defined by claim 1 wherein said added fraction contains no more then about 1 wt. % of normal paraffin hydrocarbons. 